Dealing with periodical loads and harmonics in operational modal analysis using time-varying transmissibility functions Wout Weijtjens a , John Lataire b , Christof Devriendt a , Patrick Guillaume a a Vrije Universiteit Brussel, Department of Mechanical Engineering, Acoustics and Vibration Research Group (AVRG), Belgium b Vrije Universiteit Brussel, Department of Fundamental Electricity and Instrumentation (ELEC), Belgium article info Article history: Received 17 September 2013 Accepted 15 April 2014 Keywords: Operational modal analysis (OMA) Transmissibility functions Harmonics Periodical loads System identification Damping abstract Periodical loads, such as waves and rotating machinery, form a problem for operational modal analysis (OMA). In OMA only the vibrations of a structure of interest are measured and little to nothing is known about the loads causing these vibrations. Therefore, it is often assumed that all dynamics in the measured data are linked to the system of interest. Periodical loads defy this assumption as their periodical behavior is often visible within the measured vibrations. As a consequence most OMA techniques falsely associate the dynamics of the periodical load with the system of interest. Without additional information about the load, one is not able to correctly differentiate between structural dynamics and the dynamics of the load. In several applications, e.g. turbines and helicopters, it was observed that because of periodical loads one was unable to correctly identify one or multiple modes. Transmissibility based OMA (TOMA) is a completely different approach to OMA. By using transmissibility functions to estimate the structural dynamics of the system of interest, all influence of the load-spectrum can be eliminated. TOMA therefore allows to identify the modal parameters without being influenced by the presence of periodical loads, such as harmonics. One of the difficulties of TOMA is that the analyst is required to find two independent datasets, each associated with a different loading condition of the system of interest. This poses a dilemma for TOMA; how can an analyst identify two different loading conditions when little is known about the loads on the system? This paper tackles that problem by assuming that the loading conditions vary continu- ously over time, e.g. the changing wind directions. From this assumption TOMA is developed into a time-varying framework. This development allows TOMA to not only cope with the continuously changing loading conditions. The time-varying framework also enables the identification of the modal parameters from a single dataset. Moreover, the time-varying TOMA approach can be implemented in such a way that the analyst no longer has to identify different loading conditions. For these combined reasons the time-varying TOMA is less dependent on the user and requires less testing time than the earlier TOMA-technique. & 2014 Elsevier Ltd. All rights reserved. 1. Introduction to dealing with periodical loads In mechanical engineering, operational modal analysis (OMA) is used to retrieve the modal parameters of a structure that is subjected to loads associated with its proper use [1]. For instance a bridge excited by wind and traffic loads [1] or an Contents lists available at ScienceDirect journal homepage: www.elsevier.com/locate/ymssp Mechanical Systems and Signal Processing http://dx.doi.org/10.1016/j.ymssp.2014.04.008 0888-3270/& 2014 Elsevier Ltd. All rights reserved. E-mail address: wweijtje@vub.ac.be (W. Weijtjens). Mechanical Systems and Signal Processing ] (]]]]) ]]]–]]] Please cite this article as: W. Weijtjens, et al., Dealing with periodical loads and harmonics in operational modal analysis using time-varying transmissibility functions, Mech. Syst. Signal Process. (2014), http://dx.doi.org/10.1016/j. ymssp.2014.04.008i